System for effective control of urban environment security

ABSTRACT

A security system for detecting a gunshot event. A communication link, and a number of pole units are arranged in a dense grid. Each one of the pole units includes a microphone and a signal conditioning and thresholding unit coupled to the microphone. The signal conditioning and thresholding unit outputs a detection signal in response to an event when an output signal from the microphone exceeds a peak background average. A data acquisition and signal processing unit is coupled to the signal conditioning and thresholding unit for discriminating gunshot events. The data acquisition and signal processing unit remains in a powered down stand-by mode so as to conserve energy until a detection signal is received. The data acquisition and signal processing unit includes apparatus for identifying a gunshot event by measuring an initial pulse time duration and subsequent pulse pattern features, and a communication interface coupled at an input to the data acquisition and signal processing unit. A central processor is coupled to the communication link so as to receive data from the pole units where the central processor and the pole units operate so as to detect and locate gunshot events sensed by one or more of the pole units.

This application is a continuation of application Ser. No. 08/250,743,filed May 27, 1994, now U.S. Pat. No. 5,504,717.

FIELD OF THE INVENTION

The present invention relates to the field of urban environment securitysystems.

BACKGROUND OF THE INVENTION

Given the rise in gun related violence in urban areas and the increasinginvolvement of U.S. troops in worldwide peace keeping operations therenow exists a need for a security system having capabilities to quicklydetect, locate and classify the source of gunfire with a high degree ofreliability and accuracy. The system should also provide rapidnotification to local law enforcement authorities, rescue teams ormilitary personnel, as the case may be, while aiding in locating gunshotvictims and apprehension of perpetrators. Such a system must also beable to withstand the harsh environment of an urban setting or war zone,be inconspicuous, and require a minimum of maintenance while allowingfor the easy placement and removal. The necessity for such an inventionis premised on a long felt need for rapid response by police, peacekeeping troops and rescue teams in order to significantly increase theapprehension rate of perpetrators and the survival rate of gunshotvictims.

One known device, the G.R.A.I.D.™ by 3-I, Inc. is a gunshot detector foruse in silent alarm devices. In one application, when a firearm isdischarged, a detector activates hidden cameras and notifies either thepolice or an alarm company. In another application of the G.R.A.I.D.™device, it is installed in the front grill of a police car whereby itcan detect a gunshot near the vicinity of the police car by separatingthe profile of gunshot from that of other common noises. In a situationwhere a police officer is wounded by gunfire, or discharges his weapon,the G.R.A.I.D.™ device commences a sequence to summon aid by opening thepolice transmitter, generating a series of codes distinct to each carwhich is transmitted over police radio for two seconds and is repeatedat one minute intervals until the system is reset. The transmission isreceived at the dispatch desk by decoding receiver where thetransmission is converted into the number of the sending vehicle. Thereceiver sounds an alarm and displays the number of the sending vehiclewith time and day. The receiver is reset and the vehicle contacted. Ifno contact is made with the transmitting vehicle the code will berepeated and the receiver will continue to trip once each minute. Afterfive minutes, the vehicle's horn will sound for five seconds each minuteto assist responding officers in locating the vehicle.

U.S. Pat. No. 3,341,810 to Wallen et al. entitled "Gunshot DetectorSystem", teaches a system designed to function in an extreme ambientnoise environment for detecting and distinguishing gunshot muzzle blastsand acoustic pressure waves of gun-launched projectiles passing inproximity to a target on which the system is mounted.

Wallen, et al. teaches a system wherein a plurality of detectingnetworks are placed on a high level ambient noise platform, such as ahelicopter. Utilizing pressure waves generated from an object passing athigh speed, in this case a bullet, the plurality of detecting networksis able to determine the range and distance of the source by calculatingthe intensity of the pressure wave, with muzzle indications providedonly if the distance between the detecting site and the originatingpoint of the report exceeds a predetermined amount. The Wallen et al.device determines and processes two disparate frequencies within theultra-sonic range to reduce the effect of ambient noise. The location ofthe muzzle blast is provided only if the distance between the detectingsite and the originating point of the report exceeds a predeterminedamount.

Unfortunately, prior devices as discussed above have not provenespecially effective in discriminating gunshot events from other typesof similar acoustic events such as exploding fireworks, slamming cardoors, hammer hits, and motor vehicle backfires. In contrast to theprior art, the present invention takes advantage of a new discovery thatrear acoustic detection from a gunshot event provides discriminationinformation which may be used to improve repeatable discrimination of agunshot event from other types of high acoustic events such as thoselisted above. Furthermore, systems of the prior art, are extremelylimited in range and may not effectively work at ranges over 100 feet orless.

SUMMARY OF THE INVENTION

In contrast to the prior art, a security system for detecting andreporting gunshot events for control of urban environment security isprovided. A communication link is coupled to a plurality of pole unitsarranged in a dense grid. Each of the plurality of pole units includes amicrophone, a data acquisition unit coupled to said microphone, a signalprocessor for discriminating gunshot events, and a communicationinterface apparatus coupled at an input to the signal processor. Thecommunication interface also has an output coupled to the communicationlink, where the communication interface transmits and receives data onthe communication link. A central processor is coupled to thecommunication link so as to receive data from and transmit data to theplurality of pole units. The central processor and the plurality of poleunits operate so as to detect and locate gunshot events sensed by one ormore of the plurality of pole units.

The security system of the invention provides wide-area coverage ofurban environments as opposed to a "point-defense" approach. It candetect and localize gunshots at long range and instantaneously reportsgunshot events using state-of-the-art packet radio technology.Additionally, a security system built in accordance with the presentinvention can accurately discriminate between gunshots and otheracoustic events.

Until the present invention, no security system has been provided forpermanent installation in a dense grid covering a plurality of cityblocks which can detect gunshot events and locate such events usingacoustic algorithms for pinpointing the location of the gunshot events.For the first time, the present invention provides a new and differentsecurity system capable of classifying and localizing gunfire eventsover long ranges using acoustic spectral analysis techniques and timedomain analysis techniques. Thus, in contrast to the prior art, anentire urban environment can be covered by the acoustic network employedby the invention.

The present invention is comprised of an air-acoustic detection gridcomposed of distributed sensor modules, called pole units, located ateach intersection of a city block thereby forming a dense grid ofsensing units. Each pole unit 10, as shown in FIG. 1, has a uniquetransmit identification code associated with the location of the sensoron the grid. A pole unit 10 may comprise an omni-directional microphonewith a bandwidth of 50-14,000 Hz for sensing acoustic energy. Ananalog-to-digital converter along with signal conditioning circuitry aswell as a communications transceiver which remains in "stand by" untildetection of an event or status report also may be contained within thepole unit 10. When the system detects the presence of a transient abovethe background acoustic level, and the transient exceeds a predeterminedenergy level threshold high speed digital signal processing hardware andsoftware within the sensor module is activated to differentiate gunshotacoustic characteristics from other noise sources. Transmissions from apole unit may go to a local transceiver node or base station 20 asappropriate.

Upon identification of a gunshot, buffer memory within the pole unit's10 digital modem will be filled with a time tag, pole unitidentification number, and other data necessary for gunshotdiscrimination, verification and localization. The transceiver mayinclude a digital modem and a VHF digital packet radio that connectsthrough a communications node to report gunfire to a local commandcenter.

One example of a communication system as may be used in one embodimentof the invention is described below by way of illustration and notlimitation of the invention. Transmission from the pole units 10 may be,for example, in the VHF band with transmission ranges on the order of afew miles or less. Standard RF mapping procedures may be used to locatethe pole units 10 such that complete coverage of a designated area canbe obtained, and such that the transmissions from each individual poleunit 10 can reach the node. The local nodes will be located as high asthe physical surroundings permit. Multiple node units will be located inan area to provide redundancy. The node units may be configured toautomatically retransmit the signal at higher power to the base stationat a frequency in the 900 MHz region. The range between a node and thebase station can be up to several miles. The base will receive theinformation sent from the pole unit 10 via the node almostinstantaneously, providing rapid detection and localization of theevent. Localization of the event will be performed on a centralprocessing computer located at the base station. Other types oftransmission means may be employed such as cellular telephone lines andequivalent devices

The system concept, called the System for the Effective Control of UrbanEnvironment Security (SECURES), consists of a dense grid of low-costacoustic sensing and analysis hardware and software coupled to acommunications network. SECURES will instantaneously detect, recognize,and pinpoint the location of gunfire in urban environments resulting inreduced emergency response times of up to 85%. Equipped with SECURES,local law enforcement and trauma care resources will be able to respondimmediately to gunfire, thereby dramatically increasing both theprobability of arresting the gunman and the survivability of the victim.

Other objects, features and advantages of the present invention willbecome apparent to those skilled in the art through the description ofthe preferred embodiment, claims and drawings herein wherein likenumerals refer to like elements.

BRIEF DESCRIPTION OF THE DRAWINGS

To illustrate this invention, a preferred embodiment will be describedherein with reference to the accompanying drawings.

FIG. 1 shows a high level schematic diagram of an urban security systemfor detecting and locating gunshot events as contemplated by the presentinvention.

FIG. 2 shows a schematic diagram of a base station as employed in oneaspect of an urban security system for detecting and locating gunshotevents as contemplated by the present invention.

FIG. 3 shows a schematic block diagram of a remote hardware package,herein referred to as a pole unit, as employed in one aspect of an urbansecurity system as contemplated by the present invention.

FIG. 4 shows a schematic block diagram of a signal conditioning andthreshold unit as employed in one aspect of an urban security system ascontemplated by the present invention.

FIG. 5 shows a schematic block diagram of a data acquisition and signalprocessing unit as employed in one aspect of an urban security system ascontemplated by the present invention.

FIG. 6 shows schematically a comparison of shock pulses and acousticwaveforms as detected from a handgun as measured from varying angles.

FIGS. 7A-7E show waveforms representative of rear acoustic detection forvarious weapons.

FIG. 8A and FIG. 8B together show a schematic flow diagram detailing amethod of the invention for discriminating gunshot events from otheracoustic events.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1 a high level schematic diagram of an urbansecurity system for detecting and locating gunshot events ascontemplated by the present invention is shown. The urban securitysystem includes an acoustic detection grid comprised of distributedsensor modules placed in remote hardware packages, called pole units 10.Pole units 10 may be in communication with local transceiver nodes 16for relay to a central command center or base station 20. In oneembodiment of the invention a pole unit may advantageously be located atintersections, or other locations, of a city block 14 so as to detectgunshot events 30.

In one example embodiment of the invention, time domain differencingtechniques may be employed to fix the location of a gunshot event. Usingsuch techniques, the time difference between detection times at morethan one pole unit is calculated. The time difference defines a focus ofpoints defining a hyperbola, which intersects the city street therebyfixing the location of a gunshot event at the intersection of thehyperbola and the street. If more than two pole units detect the gunshotevent more than two hyperbolas may be defined between pairs of poleunits such as hyperbola 17 and hyperbola 15 shown in FIG. 1. In suchcases, the gunshot event is fixed at the intersection of the twohyperbolas.

Now referring to FIG. 2, a schematic diagram of a base station 20 asemployed in one aspect of an urban security system for detecting andlocating gunshot events as contemplated by the present invention isshown. In one example of the present invention, the base station 20 maycomprise an RF antenna 50 coupled to a communication unit 52.Communication unit 52 is coupled to send and receive data from a basecomputer 54 which advantageously controls a display 56.

Now referring to FIG. 3, a schematic block diagram of a remote hardwarepackage, herein also called the pole unit 10, as employed in one aspectof an urban security system as contemplated by the present invention.Each pole unit 10 is comprised of a microphone 60 for sensing acousticenergy, a signal conditioning and threshold unit 62, a data acquisitionand signal processing unit 64, a communication unit 66 and a powersupply 370. The communication unit 66 may include a transceiver 68 and atransmit buffer 512. The transceiver 68 may advantageously include adigital modem 72 and a VHF digital packet radio transmitter 70 thatconnects through a communications node, to report a gunfire incident toa local command center which includes the local transceiver node.

In one preferred embodiment, each pole unit 10 may advantageously have aunique transmit identification code indicative of the location of thesensor in the grid. When the presence of a high-intensity transient suchas is produced by a gunshot is sensed, high-speed digital signalprocessing hardware and software within the sensor module differentiatesgunshot impulsive transients from other noise sources. In one mode ofoperation, upon identification of a gunshot event, buffer memory withinthe pole unit's 10 digital modem may be loaded with a time tag notingthe time of the event, pole unit identification number, an acousticsignal and any other data necessary for gunshot verification andlocalization.

In one embodiment using RF communication, for example, modem buffer datamay be formed into digital communications packets and transmitted viaVHF radio to local nodes for relay to the base station. At the basestation, the data will be unpacked. Data from multiple sensors may beused to perform time-difference fixing to localize the gunfire eventwithin the city block and support additional false alarm discriminationas required.

In one embodiment of the invention, the microphone 60 may comprise anomni-directional microphone with very wide bandwidth response to sensethe acoustic energy. Good acoustic response over a band ranging from50-14,000 Hz may be used in detecting the energy components required forclassification of gunshot spectra in the presence of other potentialtransient false alarms, while still being resilient to the adverseimpact of atmospheric absorption on the transmitted energy.

In one example, the data acquisition unit 62 may include ananalog-to-digital (A/D) converter, analog amplification and filteringcircuitry for signal conditioning. This circuitry, similar tohigh-fidelity sound recording, may include additional spectralde-emphasis if required.

Note that battery power, as opposed to local service is an optionaldesign alternative. The power supply may use solar cells for primarypower and a lead-acid battery for secondary power. In one example, thelead-acid battery may be continuously charged from the solar cells by atrickle charging circuit that supplies a current greater than theaverage current dissipated by the units. The lead-acid battery, whilenot the smallest type of battery that can be used, is economical, andhas a fairly broad range of operating temperatures. Other,special-purpose battery types may be advantageously used such as lithiumcells. Alternative designs such as tapping into local service power mayalso be used in some examples of the invention.

Referring now to FIG. 4, a schematic block diagram of a signalconditioning and threshold unit as may be employed by one embodiment ofthe invention is shown. The microphone signal on line 61 is passedthrough an initial variable gain stage or amplifier 82 to adjust thesignal to a level appropriate for processing. The adjusted signal isoutput onto line 84 and then switched through switch 86.

Switch 86 may switch the amplified signal through to switch 194 whichmay be in a position to pass the signal through on line 96 to comparator98. Optionally, a rectifier 90 may be employed. If the rectifier is usedthe signal may optionally be routed by switch 86 onto line 88 throughthe rectifier 90 which passes the rectified signal through to line 92and subsequently through switch 194. Use of rectifier 90 may be helpfulif needed for optimizing peak detection by a peak background levelaverager shown here as peak background level averager 124. The signal issubsequently passed on line 96 to comparator 98 where it is compared toa selective reference voltage which is set to trigger the comparator fortransients greater than a preselected level, e.g., 100 decibels.Comparator 98 provides a detection output 100 into AND logic 102. Alsoincluded in the signal conditioning and threshold unit are an averagerrise time constant circuit 130 connected by line 128 to the peakbackground level averager 124 together with an averager decay timeconstant circuit 132 which is also connected to the peak backgroundlevel averager 124 through line 126. The peak background level averageroutputs a peak background level signal on line 140 to a secondcomparator 108. The second comparator 108 also receives a referencesignal on line 110 from divider 112 which receives the amplified signalon line 122 and has its ratio N set by unit 116 through line 114. Inoperation, the averager rise time constant and averager decay timeconstant operate together with the peak background level averager inorder to provide a peak background average signal in a dynamic fashion.In one example, the peak background averager comprises a peak detectorhaving a decay time constant of about 100 ms. As background transientscreate peak signals above the current average background level, the peakaverager rises to the higher level and begins to decay resulting in asaw tooth-like output. The output is then fed to a low pass filterhaving a rise time constant of about 3 ms. Thus, since a gunshot occursin about 1 ms, gunshot events will stand above the average backgroundlevel and not be tracked by the peak detector. The 3 millisecond timeconstant is thus customized to pass a gunshot event.

In one example of the invention, the averager rise time constant may beabout 3 msec while the averager decay time constant may be about 100msec. The peak background level averager applies the rise time constantand decay time constant to acoustic signals from loud background eventssuch as busses or fire engines passing a pole unit, for example. Keepingsuch a running background average noise level allows the signalconditioning and threshold unit to discriminate such transientbackground events from candidate gunshot events. In one example of theinvention, the ratio N may be set to a magnitude of about 5, forexample, so that if the new event is 5 times louder than the averageevent, it will cause the comparator to trigger a signal on line 106 intoa second input of the AND logic 102. In operation, if the signal has apeak above the reference signal on line 110, comparator 108 will triggerand if the signal exceeds the minimum reference voltage set in circuit118, comparator 98 will also trigger, thus yielding a detection pulsefrom AND logic 102 on line 104. The detection pulse is used to activatedownstream processors and circuits.

An amplified signal on line 150 is also provided to analog-to-digitalconverter 94 where it may be converted into a digital format using, forexample, 16 bit A/D converted sampling at a rate appropriate for thefrequency response of the microphone 60. A high sampling rate may beused to obtain sufficient samples for spectral analysis of the rise timeof an explosive transient wave form caused by a gunshot. Sixteen bits ofanalog quantitization provides approximately 14 bits of accuracy undertypical operating conditions. Such a design has approximately 84 dB ofdynamic range.

Referring now to FIG. 5, a schematic block diagram of a data acquisitionand signal processing unit 64 as employed by one example embodiment ofthe invention is shown. The data acquisition and signal processing unitremains in a powered down "stand-by" mode to conserve energy until adetection signal is received on line 104. The system of the inventiongenerally is activated on an incremental basis to conserve energy. Onlythose circuits needed for processing are energized on an as neededbasis. The data acquisition and signal processing unit 64 advantageouslycomprises analog-to-digital convertor, a processor 502, associatedmemory 504 and bus 506. In operation, the processor 502 receives datafrom the signal conditioning and thresholding unit 62 and processes itto detect and classify the transient events of interest. Dataacquisition buffers 510 in the unit allow the processor 502 to continueprocessing while new input data is being received from the signalconditioning and thresholding unit 62, and detection data is being sentto the communications unit 66 on line 513. The processor's program maybe stored in memory 504. Memory 504 may advantageously comprise anon-volatile memory device that retains information in the event of apower failure.

In one embodiment of the invention, digitized microphone data is passedto the processor 502 via the multi-port data acquisition buffer 510 thatis mapped into the processor memory space. The processor 502 processesthe data to detect transients and to classify them as gunfire ornon-gunshot events. Upon a decision of positive classification, data isplaced into a transmit buffer 512 of the communications unit 66. In oneembodiment, one example of a useful processor is a DSP chip which may bea TI Model TMS320C31™ 32 bit floating point processor rated at 33 MFLOPSor a TI Model TM5320C25™.

Classification of transient events from gunshots may be performed usinga variety of signal processing techniques, as appropriate. Suchtechniques may include energy estimation, transient shape analysis,rise-time derivation, spectral analysis, time domain analysis andreplica correlation. State-of-the-art classification algorithms such asBayes algorithm, nearest neighbor algorithms, and neural networks thatoperate over these feature sets may be applied as appropriate. Inaddition, post processing techniques that restore high frequencycomponents suppressed by atmosphere absorption have been devised toimprove probability of correct classification.

The apparatus and method of the invention contemplate that alldiscrimination analysis or a portion of the discrimination analysisincluding the classification of transient events may be done in a hostcomputer located at the base station. The host computer will haveinformation from all pole units reporting the same event and may be ableto incorporate that information into making a decision as to whether ornot to report a certain event. The host computer may also receivedetected acoustic signal waveforms from the pole units and be able toprocess those waveforms directly. In one example of the invention, it iscontemplated that the host computer may be connected to an audio speakerfor replay of the acoustic event through the speaker so that adispatcher at the base station may hear the event and make a decisionbased upon the audio playback.

Referring again to FIG. 3, the communications unit 66 may comprise anantenna 69, a transmit buffer 512, a transceiver 70, a packet box 71,and a power supply. The antenna 69 may advantageously be a helicalall-weather resonant flexible unit designed for operation in the 900 MHzband. It may be attached to the pole unit 10, Node, and Base units usinga waterproof connector.

In one embodiment of the invention, the transceiver may be a crystalcontrolled transceiver. Alternatively, a spread spectrum, FSK orfrequency synthesized transceiver may be used. An example of a crystalcontrolled transceiver is the MAXON DM-0500™ series of Telemetry/Datamodule radios. An example of a frequency synthesized receiver is theMOTOROLA RNet Telemetry Radio™ series.

The packet box 71, sometimes called a Terminal Node Controller (TNC) isavailable from several manufacturers, including Kamtronics, HALCommunications, or AEA. Any standard TNC already has the majority of thecharacteristics required for SECURES. Special firmware will be requiredfor any selected TNC. The TCP/IP protocol will be installed, and aspecial circuit will be added which will not allow any node changes.When the radio is powered up, the TNC will automatically be in theCONVERSE mode, with the TCP/IP networking capability operational, andnot revert to the COMMAND mode which is the standard TNC architecture.The TNC may be about the size of typical cigarette package.

Referring now to FIG. 6, differences in acoustic waveforms as measuredfrom various angles are generally illustrated. A gun 600 having fired abullet 602 is shown together with waveforms 604, 606, 608 and 610. Ithas now been discovered that, in contrast to the approaches taken by theprior art, gunshot signatures of a uni-directional source such as ahandgun 600 may be discriminated from other events using rear acousticdetection. A bullet 602 generates a shock pulse 605, 607 as measuredfrom the front and accompanied by a gas generated shock pulse from thegun itself in the front as measured from the front of the gun as shownin waveforms 604 and 606. Waveforms 604 represents a gas generated shockpulse as measured along an axis which is displaced 0 degrees from theaxis of the gun barrel. Waveform 606 shows another measured gasgenerated shock pulse as measured at an angle of about 30° from the axispassing through and along the gun barrel of gun 600. Contrast thosewaveforms with the waveforms measured from the rear of the gun 600,namely rear acoustic detection signals 608 and 610.

Waveform 608 represents a waveform as measured at an angle of about 150°as referenced to the axis passing to the front of the gun barrel andwaveform 610 is measured at about 180° or directly rearwardly of thefront of the gun barrel. Note that waveforms 608 and 610 display apattern of peaks or oscillations, not displayed in the gas generatedshock pulses measured generally from the front of the fired gun 600. Inaddition to features derived from the waveform, the present inventionexploits this newly discovered phenomenon in order to build adiscrimination method for differentiating gunshot events from other nongunshot events such as firecrackers exploding, balloon pops, etc.

Referring now to FIGS. 7A-7E, a number of waveforms measured using therear acoustic detection method of the invention are shown for variousfirearm types. The waveform data for each type of weapon was measuredfrom a distance of about 75 meters. FIG. 7A shows a rear acousticdetection waveform for a 12 gauge shotgun. FIG. 7B shows a rear acousticdetection waveform for a .308 rifle. FIG. 7C shows a rear acousticdetection waveform from a 9 mm block. FIG. 7D shows a rear acousticdetection waveform from a .45 acp. FIG. 7E shows a rear acousticdetection waveform from a .38 special. Such waveforms may be held inmemory in digital form for comparison with detected signals.

Gunshot signatures observed behind a gun have characteristics thatdistinguish short and long barrel weapons and possibly classes of shortand long barrel weapons. This signature is dominant when observed behindthe gun, but is superimposed on the acoustic pulse observed in theforward direction too. In the illustrated case, it represents arelatively small fraction of the total acoustic energy. The signaturemay be extracted by pattern recognition methods to allow gunclassification to be effective for most observation angles.

Referring now to FIG. 8A and FIG. 8B, a flow diagram of the method ofthe invention for discriminating gunshot events from other acousticevents is shown. It is not necessary for all of this analysis to beperformed in the pole unit. It may be done in the host computer. Theonly requirement is that the pole unit discriminator is effective enoughto minimize RF transmission and to extend battery life.

At process step 802, an acoustic signal is measured. The acoustic signalis introduced into the analog trigger circuitry comprising the signalconditioning and thresholding unit 62 at process step 804. If the signalconditioning and thresholding unit outputs a detection signal, the eventduration is measured at process step 806. If the event duration is notless than a first predetermined time period, for example, about 6milliseconds, the process is routed to process step 808. If the eventduration is less than the first predetermined time period, the processflows to process step 816.

Block 808 represents a state wherein further analysis is required todetermine whether the acoustic event was a gunshot, a fire cracker, or.22 blank with multipaths as opposed to an impulsive event such as adoor slam, backfire, or hammer hit with or without multiple paths. Theprocess then proceeds to step 810 wherein an analysis of a secondpredetermined time period, for example, a 50 msec time period followingdetection of the event is initiated. At process step 812 the analysis ofthe signal detected during the second predetermined time period iscarried out. Three feature analysis methods are applied to the waveformwithin the second predetermined time period following detection.

The majority of impulse events from which to discriminate gunshots havecharacteristics of a mechanical system associated with them thatresonates. For example, in hitting metal hammers together, sharpacoustic impulses are generated with rise times comparable to gunshots,but the hammer resonates for a period of time significantly longer thanthe duration of a gunshot. However, in the case where a gunshot is firedamong buildings or other obstacles, multiple echoes may follow thedirect transmission of the signal to the microphone. In such a case,analysis is required to distinguish a gunshot with echoes from anothersource with longer duration. The following three methods may be used toprocess the waveform:

Low Amplitude Gap Feature: This analysis keys on the fact that echoesarrive from a limited number of discrete paths within about the first 50msec, for example, of the detection. Echoes, unlike other noise sources,will have gaps between these arrivals. The gap feature is a statisticalmeasure of the number and duration of such gaps.

Frequency Content Feature: This feature is a measure of the percent ofthe waveform that is above a predetermined frequency and is a gooddiscriminator for impulsive events such as metal clanks, backfires, hoodslams and glass breakage.

Periodicity Feature: Most noise sources have a dominant resonantfrequency or repetitious nature. Detection of a dominant frequency andits harmonics is a good discriminator for events such as metal clanks,hood slams, backfires, sirens, etc.

If the feature methods rule out a gunshot event at process step 812, theprocess flows to process step 814 where the event is classified as a nongunshot event.

If the features do not rule out a gunshot event, the process flows toblock 816, where a distinction is made between a gunshot, firecracker,balloon pops or .22 blank without multiple paths. Block 816 may also beentered via step 806 as discussed above. A decision must be made betweenthe choices delineated in block 816. The process flows to step 818 wherea first deposit of pulse duration of greater than a third predeterminedtime period, for example, 0.2 msec, is measured. If no first positivepulse duration of greater than the third predetermined time period isdetected, the process flows to step 820 where the acoustic event isclassified as a small fire cracker, balloon pop, .22 blank or othernon-gunshot event. If the measurement at process step 818 proves greaterthan the third predetermined time period, the process flows to processstep 822 wherein a discrimination must be made between a gunshot and alarge firecracker, such as an "M80". To discriminate between a gunshotand a large firecracker the process flows to step 824 where a multiplepulse pattern from a signal taken from behind a gun is compared to thedetected signal. If the multiple pulse pattern compares withinpreselected tolerance levels against preselected patterns such as thoseshown in FIGS. 7A-7E, the process flows to step 828 where it isclassified as a gunshot. Depending upon pattern matching of variousstored gunshot patterns, such as shown in FIGS. 7A-7E, the gunshot mayfurther be classified into a weapon type such as a pistol 830 or a rifleat step 832. If the pattern of the detected gunshot signal does notcompare within preselected tolerance ranges with the stored rearacoustic detection patterns, the acoustic event will be classified as alarge firecracker or a gunshot.

The invention has been described herein in considerable detail in orderto comply with the Patent Statutes and to provide those skilled in theart with the information needed to apply the novel principles and toconstruct and use such specialized components as are required. However,it is to be understood that the invention can be carried out byspecifically different equipment and devices, and that variousmodifications, both as to the equipment details and operatingprocedures, can be accomplished without departing from the scope of theinvention itself.

What is claimed is:
 1. A security system for detecting a gunshot event,wherein a gunshot event produces an acoustic signature including aninitial pulse and subsequent pulse pattern, the security systemcomprising:(a) a communication link; (b) a plurality of pole unitsarranged in a dense grid, wherein each one of the plurality of poleunits includes,(i) a microphone for providing an output signal inresponse to an acoustic event, (ii) a signal conditioning andthresholding unit coupled to said microphone wherein the signalconditioning and thresholding unit outputs a detection signal when theoutput signal exceeds a peak background average signal, where the peakbackground average signal is dynamically determined, (iii) a dataacquisition and signal processing unit for discriminating gunshot eventsconnected to receive the detection signal, wherein the data acquisitionand signal processing unit remains in a powered down stand-by mode so asto conserve energy until the detection signal is received, and whereinthe data acquisition and signal processing unit includes means foridentifying a gunshot event by measuring an initial pulse time durationand a plurality of subsequent pulse pattern features in the detectionsignal, and, (iv) a communication interface coupled at an input to thedata acquisition and signal processing unit, the communication interfacealso being coupled to the communication link, where the communicationinterface transmits and receives data on the communication link; and (c)a central processor coupled to the communication link so as to receivedata from the plurality of pole units where the central processor andthe plurality of pole units operate so as to detect and locate gunshotevents sensed by one or more of the plurality of pole units.
 2. Thesecurity system of claim 1 wherein the communication interface comprisesa modem and a receiver/transmitter coupled to the communication link. 3.The security system of claim 1 wherein the signal conditioning andthresholding unit comprises:(a) an amplifier coupled to the microphonefor receiving an acoustic signal from the microphone, the amplifierhaving an amplified output for carrying an amplified acoustic signal;(b) a first comparator having a first input coupled to the amplifiedoutput; and (c) a reference signal coupled to a second input of thefirst comparator wherein the first comparator outputs a detection signalif the amplified detection signal exceeds the reference signal.
 4. Thesecurity system of claim 1 wherein the data acquisition and signalprocessing unit further comprises a digital modem including means forfilling the digital modem with a time tag and a pole unit identificationnumber.
 5. The security system of claim 1 wherein the data acquisitionand signal processing unit further comprises means for detecting thepresence of a transient above the background acoustic level, wherein thetransient exceeds a predetermined energy level threshold.
 6. Thesecurity system of claim 5 wherein the data acquisition and signalprocessing unit further comprises high speed digital signal processinghardware and software that is activated to differentiate gunshotacoustic characteristics from other noise sources.
 7. The securitysystem of claim 1 for detecting a gunshot event wherein the signalconditioning and thresholding unit includes a peak background levelaverager, where the peak background level averager applies a rise timeconstant and a decay time constant to acoustic signals where theaverager rise time constant and averager decay time constant operatetogether with the peak background level averager in order to dynamicallyprovide the peak background average signal so as to allow the signalconditioning and threshold unit to discriminate transient backgroundevents from candidate gunshot events.
 8. A method for sensing a gunshotevent comprising the steps of:(a) sensing a transient acoustic event;(b) producing a first signal representative of the transient acousticevent; (c) dynamically producing a second signal representative of theaverage peak background level; (d) responding to the first signal bypowering on a means for determining whether the transient event exceedsthe average peak background level by a specified ratio and whether thetransient event exceeds a predetermined energy level threshold; (e) ifthe conditions of step (d) are true, then determining whether thetransient event exhibits an event duration less than a predeterminedtime period; and (f) if condition of step (e) is true, then initiatingRF transmission of a message from an RF transmitter that is in a poweredoff state unless transmitting.
 9. The method of claim 8 wherein the stepof dynamically producing a second signal representative of the averagepeak background level further comprises the steps of applying a risetime constant and a decay time constant to acoustic signals where therise time constant and the decay time constant dynamically produce apeak background average signal.